PatchTheCEVCVE-2026-8833: Improper neutralization of HTML-encoded characters in the URL validation function in Checkmk <2.5.0p5, <2.4.0p
22
Routine
MEDIUM 5.4DOWNGRADE
CVE-2026-8833 is a medium-severity flaw (CVSS 5.4) affecting checkmk. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.
Improper neutralization of HTML-encoded characters in the URL validation function in Checkmk <2.5.0p5, <2.4.0p31, <2.3.0p48, and all 2.2.0 versions allows an authenticated user to bypass URL validation and inject malicious URLs such as javascript: URIs, resulting in cross-site scripting when another user interacts with the crafted link.
Why this priority
- No elevated risk signals yet
No fix is published yet: follow the downgrade or mitigation steps.
Risk facts
| CVSS score | 5.4 (Medium) |
| Attack vector | Network (remote) |
| Attack complexity | Low |
| Privileges needed | Low |
| User interaction | Required |
| EPSS exploit chance | 0.2% (top 86%) |
| CISA exploited list | No |
| Weakness | CWE-79 |
| Published | 2026-06-08 |
| Last modified | 2026-10-23 |
Publisher (CNA): Checkmk GmbH · Vendor: checkmk
Am I affected?
Timeline
- Published CVE record published
- Updated Record last modified
checkmk checkmk DOWNGRADE
Affected: 2.2.0 · 2.2.0 b1 · 2.2.0 b2 · 2.2.0 b3 · 2.2.0 b4 · 2.2.0 b5 · 2.2.0 b6 · 2.2.0 b7
Fixed in: no fixed release known
Safe to downgrade to: a release before 2.2.0
1. Check installed version
apt-cache policy checkmk || rpm -q checkmk || checkmk --version
2. No fixed release: DOWNGRADE to a release before 2.2.0 (Debian / Ubuntu)
apt-cache madison checkmk
sudo apt install checkmk=<VERSION_BEFORE_2.2.0>
sudo apt-mark hold checkmk
2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list checkmk
sudo dnf downgrade checkmk-<VERSION_BEFORE_2.2.0>
sudo dnf versionlock add checkmk
Downgrade risk
Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.
After downgrade
Restart the service, and keep the package held until a fixed release ships.
Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
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